This invention relates to apparatus for depositing a material such as diamond on a substrate by chemical vapour deposition.
Methods of depositing material such as diamond on a substrate by chemical vapour deposition (hereinafter referred to as "CVD") are now well established and have been described extensively in the patent and other literature. Where diamond is being deposited on a substrate, the method generally involves providing a mixture of hydrogen or oxygen gas and a suitable gaseous carbon compound such as a hydrocarbon, applying sufficient energy to that gas to dissociate the hydrogen into atomic hydrogen or the oxygen into atomic oxygen and the gas into active carbon ions, atoms or CH radicals and allowing such active species to deposit on the substrate to form diamond. Dissociation of the gases can take place by a variety of methods.
One such method is the use of a hot filament. In this method, the temperature of the filament is about 2000.degree. C. and the substrate on which the diamond growth occurs is at 600.degree. to 1100.degree. C.
Apparatus for depositing diamond or other material on a substrate by chemical vapour deposition using the hot filament method will generally include a closed reaction chamber having at least one gas inlet and at least one exhaust means. The apparatus will further include means for supporting the substrate and resistance heating means in the form of electrically conductive filaments across which the gas is passed to cause it to dissociate. In use, the chamber will generally be maintained at a pressure below atmospheric.
One of the inherent practical problems with the hot filament deposition method is the deformation and fragility of the filament during repeated heat cycles. Various solutions to prevent filament deformation and sagging have been suggested. Most of these suggestions are based on the concept of tensioning the filaments so as to take up the expansion during heating. There are disadvantages with the tensioning technique. The filament stretches under tension at high temperatures creating thinner sections in the filament which give rise to hot spots and eventual breakage. Further, the technique requires complicated mechanical assemblies in order to tension filament wires which are then heated by high electric currents.
South African Patent No. 90/5696 provides resistance heating means in the form of a plurality of vertically extending linear, electrically conductive filaments prestressed to curve in a single plane substantially equidistant from and parallel to the substrate on which deposition is to take place. The filaments are secured at each end to a pair of electrodes.
Another frequent problem is the short life of the filaments if heat cycles are involved in the method. After being heated and exposed to the growth atmosphere most filament materials carburise and become fragile. In conventional filament holding assemblies, this will invariably result in filament breakage during the cooling cycle.